p55 is a palmitoylated peripheral membrane phosphoprotein of human red blood cells. p55 forms a complex with protein 4.1 and glycophorin C. This protein complex is missing in genetically abnormal red blood cells with altered shape and markedly reduced membrane stability. Although the precise physiological role of p55 is unknown, interest in this component has been heightened by its similarity to the Drosophila discs-large tumor suppressor protein. p55 and recently cloned Hdlg (human B lymphocyte homologue of the Drosophila discs-large) are members of a growing family of proteins called MAGUKs which are believed to play a role during signal transduction at the membrane-cytoskeleton interface. In this project, we wish to study the interaction of human p55 and Hdlg with cytoskeletal protein 4.1, and determine whether these interactions are regulated by guanine nucleotides and phosphorylation. The long-term goal of these studies is to test the hypothesis that interaction between MAGUKs and protein 4.1 homologues are widespread and physiologically important. Using protein binding assays developed in this project, we will determine if other MAGUKs such as Drosophila Dlg, tight junction Z0-1 and C. elegans LIN-2 proteins also bind to protein 4.1. In collaboration with Dr. Anderson, we will test whether tyrosine phosphorylation of p55 and Hdlg occurs in mammalian cell systems under the conditions developed in his laboratory. In summary, the interactions between MAGUKs and protein 4.1 may emerge as a generally important site for signal transduction and membrane-cytoskeleton linkage.